For my money, Juliet is practical, brave, and headstrong. Here are some quotes to get you started on any one of these characterizations.
In the opening of the balcony scene, she displays her practical mind when, as Romeo stands below her window telling the audience how much he loves her, she is considering the huge obstacle to that love -- he is a Montague and her sworn enemy:
O Romeo, Romeo, wherefore art thou Romeo?
Deny thy father and refuse thy name.
Or, if thou wilt not, be but sworn my love
And I'll no longer be a Capulet.
Juliet's practical turn of mind is revealed in her first words in the famous balcony scene. She speaks not about how dreamy-eyed in love she is, but rather the practical events that must happen in order for them to be together.
A good example of her bravery comes quite late in the play. She has just been kicked out of her home by her father (if she refuses to marry Paris) and had her closest confidant, The Nurse, tell her she should marry Paris and forget Romeo. She decides to go completely alone to the Friar and beg him for remedy. If he has none, she vows to kill herself or undergo any other gruesome act, rather than marry Paris:
O, bid me leap, rather than marry Paris,
From off the battlements of any tower
Or walk in thievish ways, or bid me lurk
Where serpents are. Chain me with roaring bears,
Or hide me nightly in a charnel-house.
Ultimately, she commits a huge act of bravery, when she drinks the potion that the Friar gives her, not knowing if it will kill her or merely cause her to seem dead.
Her headstrong nature can actually be seen in the events I have described above, and in her determination to marry Romeo, even though he is her family's enemy. She is the one that pushes for marriage, and one wonders if Romeo would have had the gumption without her headstrong presence behind him.